Project Summary: In the human condition, a chronic, full thickness rotator cuff tear without surgical repair results in a permanent defect in the tendon, and the unloaded muscle progresses to severe and irreversible atrophy and fibrosis. There is no existing animal model that replicates these critical aspects of the human condition. In the rat shoulder model, the tendon defect undergoes spontaneous healing and the muscle changes are reversible. None of the existing large animal shoulder models have included capsulectomy, thus the tendon injury has been extra-articular and undergoes spontaneous healing. Therefore, we argue that a new animal model is needed, one that mimics the human condition with regard to joint capsule injury, the absence of spontaneous healing or scar formation, the presence of muscle atrophy, stiffening, and fat accumulation, and the ability to control post-operative rehabilitation. Thus it is the aim of this proposal to develop and validate a clinically relevant animal model of chronic rotator cuff injury and repair. We will perform infraspinatus tendon release together with joint capsule resection in a canine shoulder model. We will perform three separate studies, which we believe are the minimum necessary to address our aim. The primary objectives of these studies are to: (1) validate the absence of spontaneous healing after tendon release and joint capsule resection, (2) determine the influence of post-operative immobilization by casting on an acute tendon repair, and (3) determine the repairability and reversibility of a six-week chronic tendon injury. Relevance: Chronic rotator cuff tendon tears are a frequent cause for morbidity in the adult human population. Surgical repair of chronic tears is indicated when conservative treatment has failed to improve the patients' symptoms. However, repairs of large chronic cuff tears fail to heal in 20-95% of cases, posing a significant clinical problem. Successful development of this model would lead to important studies of critical factors thought to affect the outcome of this common clinical problem such as surgical technique, tear morphology and timing. [unreadable] [unreadable] [unreadable]